The findings come from a UC Berkeley study, which also found that only four of these top web sites using Flash cookies mention them in their privacy policies. From Wired:

Unlike traditional browser cookies, Flash cookies are relatively unknown to web users, and they are not controlled through the cookie privacy controls in a browser. That means even if a user thinks they have cleared their computer of tracking objects, they most likely have not.

If you're serious about your online privacy and thought blocking traditional browser cookies was doing the job, you may want to consider trying one of the tools above and hitting up the Wired post for the full rundown.